Because silicon has a high refractive index, a single mode silicon waveguide exhibits sub-micron mode size. The observed loss is high when an optical mode couples to a single mode fiber because of a large mode mismatch. Grating couplers have large mode profiles, but their bandwidth is limited and they are usually polarization dependent. Edge couplers have broad bandwidth with small polarization dependent loss. However traditional inverted-taper-based edge couplers usually are constructed to work with lensed fiber because their mode sizes are usually around 3 μm. This imposes difficulties in packaging, because it requires accurate alignment between the edge coupler and the lensed fiber. To match to 10 μm mode field diameter (MFD), people have built SiO2 waveguides with Si/SiN waveguide inside the SiO2, but this method requires a silicon undercut with wet etch to avoid substrate loss which is costly, risky and time-consuming.
Also known in the prior art is Assefa et al., U.S. Pat. No. 7,738,753, issued Jun. 15, 2010, which is said to disclose an optoelectronic circuit fabrication method and integrated circuit apparatus fabricated therewith. Integrated circuits are fabricated with an integral optical coupling transition to efficiently couple optical energy from an optical fiber to an integrated optical waveguide on the integrated circuit. Layers of specific materials are deposited onto a semiconductor circuit to support etching of a trench to receive an optical coupler that performs proper impedance matching between an optical fiber and an on-circuit optical waveguide that extends part way into the transition channel. A silicon based dielectric that includes at least a portion with a refractive index substantially equal to a section of the optical fiber is deposited into the etched trench to create the optical coupler. Silicon based dielectrics with graded indices are also able to be used. Chemical mechanical polishing is used finalize preparation of the optical transition and integrated circuit.
Other relevant publicly available documents that describe the prior art include: M. Wood, P. Sun, and R. M. Reano, “Compact cantilever couplers for low-loss fiber coupling to silicon photonic integrated circuits,” Opt. Express, vol. 20, no. 1, p. 164, 2012; L. Chen, C. R. Doerr, Y. Chen, and T. Liow, “Low-Loss and Broadband Cantilever Couplers Between Standard Cleaved Fibers and High-Index-Contrast Si3N4 or Si Waveguides,” IEEE Photonics Technol. Lett., vol. 22, no. 23, pp. 1744-1746, 2010.; L. Jia, J. Song, T.-Y. Liow, X. Luo, X. Tu, Q. Fang, S.-C. Koh, M. Yu, and G. Lo, “Mode size converter between high-index-contrast waveguide and cleaved single mode fiber using SiON as intermediate material,” Opt. Express, vol. 22, no. 19, p. 23652, September 2014; and R. Takei, E. Omoda, M. Suzuki, S. Manako, T. Kamei, M. Mori, and Y. Sakakibara, “Low-loss optical interlayer transfer for three-dimensional optical interconnect,” in Proceedings of 10th International Conference on Group IV Photonics (Seoul, South Korea, 2013), pp. 91-92.
There is a need for improved couplers for optically interconnecting chips and optical fibers.